This invention relates to disc drive spindle assemblies, and more particularly, to a disc drive spindle assembly which can be assembled without the use of adhesives.
Computer information is often written to and read from a rotating recording medium. The recording media is generally one or more magnetic discs upon which information is encoded by a transducer. Discs are usually rotated at a relatively high speed and a relatively constant velocity during computer use, rather than repeatedly starting and stopping each time information is transmitted. To achieve this high speed, relatively constant velocity disc rotation, electrically-powered spindle motors are used. The read/write transducer is moved radially with respect to the axis of disc rotation, such that the entire surface of the disc(s) may be accessed.
Spindle motors with separate rotor assemblies and stator assemblies are commonly used for disc drive applications. The rotor generally carries a multipolar magnet, which is adhesively mounted about a lower periphery of the rotor. The stator typically includes a plurality of radially oriented electromagnets, with the polarity of the electromagnets alternated based on the location of the multipolar magnet in the rotor. The multipolar magnet responds to the alternating magnetic field to rotate the rotor and disc(s). While the polarity of the electromagnets in early disc drives was controlled by a brush type contact, the direction of current and polarity of the electromagnets is now generally controlled by back emf within electromagnet coils created by the rotating multipolar magnet. The separateness of the rotor and stator allow a low height profile, and interchangeability of motor sub-assemblies.
In assembly of this separate rotor/stator configuration, the stator is adhesively mounted to a disc drive base plate. Then the rotor assembly is adhesively mounted to ball bearings which are in turn adhesively mounted about a cylindrically-shaped shaft. The shaft is mounted to the base plate such that the multipolar magnet is in proper position relative to the stator. After the spindle motor has been assembled, the disc(s) may be attached to the rotor.
The stator assembly in the spindle motor is one of the contributors to the acoustic noise characteristics generated by the spindle motor. Some of the noise caused by the stator assembly is due to loose magnetic wires in the coils of the stator electromagnets. Wires in the coil tend to move due to the alternating current and alternating magnetic fields encountered. Additionally, exposed metal within the stator could be corrosive. To reduce acoustic noise in the electromagnetic coils and avoid corrosion in the stator, stators are coated with an epoxy adhesive layer.
In addition to the bearing/rotation relationship described above, a magnetic fluid seal is made between the rotor and the shaft. The magnetic field is held as part of a magnetic fluid seal on the rotor, and is retained in contact against the shaft by the magnetic field around the shaft. The magnetic fluid provides a return flow path for magnetic flux between the rotor and the shaft. The magnetic fluid also allows electrical current (known as electro-static discharge, or "ESD") to be readily transmitted between the shaft and the rotor and thus avoid the build up of potentially hazardous voltage differentials. Improper build-up and release of ESD could adversely effect the magnetic recording media. Typically the magnetic fluid seal assembly is held to the rotor through the use of adhesive.
The requirements for the adhesives to hold these various component parts together are particularly demanding and pose a difficult manufacturing and design problem. The adhesive must have considerable structural strength over a wide temperature cycle (up to from -40.degree. C. to +70.degree. C.). The adhesive must retain structural strength over the life of the disc drive. The adhesive used must cure quickly and completely and be easy to dispense, to avoid unnecessary delay during spindle assembly. Additionally, the adhesive must have low outgassing and must be dispensable without contacting improper surfaces, to maintain the necessary cleanliness and performance of the disc drive.
To satisfy all these requirements there are very few adhesives which are practically available. The adhesives available are expensive, and the cost of installation, including process time and tooling, is high. Even when coated with epoxy adhesive, the non-smooth profile of the wire coil tends to attract contaminants such as dust particles, adversely affecting the performance of the disc drive. Accordingly, a spindle assembly is desired which will avoid the costs and problems of adhesives while still allowing proper attachment of the components and minimizing the acoustic and dust collecting problems of the stator assemblies.